Optical communications systems of the above type are employed in broad-band distribution networks, e.g. in integrated services networks, and they use optical fibers as a broad-band transmission medium. Examples of these systems and of their applications are described by R. Oishansky, V. A. Lanzisera and P. M. Hill in the paper: "Subcarrier Multiplexed Lightwave Systems for Broad-Band Distribution", Journal of Lightwave Technology, Vol. 74, NO. 9, Sep. 1989, pages 1329 ff.